Advances in Geosciences

The following sections are included:

• EDITORS

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• CONTENTS

We present the basics of the wave hop theory to compute the amplitude and phase of the VLF signals. We use the Indian Navy VTX transmitter at 18.2 kHz as an example of the source and compute the VLF propagation characteristics for several propagation paths using the wave-hop theory. We find the signal amplitudes as a function of distance from the transmitter using wave hop theory in different bearing angles and compare with the same obtained from the Long Wave Propagation Capability (LWPC) code which uses the mode theory. We repeat a similar exercise for the diurnal and seasonal behavior. We note that the signal variation by wave hop theory gives more detailed information in the day time. We further present the spatial variation of the signal amplitude over whole of India at a given time including the effect of sunrise and sunset terminator and also compare the same with that from the mode theory. We point out that the terminator effect is clearly understood in wave hop results than that from the mode theory.

VLF chorus emissions recorded at Indian Antarctic station, Maitri (Lat = 70°46′S, Long = 11°50′E, L = 4.5) during a geomagnetic quiet period on 3 Feb. 2001 is reported. The detailed spectral analysis of recorded chorus emissions shows that each chorus element originates from the upper edge of the underlying hiss band. The observed mean chorus element parameters are as follows: lower band frequency f_min = 3.1 kHz, upper band frequency f_UB = 4.8 kHz, frequency sweep rate df/dt = 2.1 kHz/s and repetition period T = 1 s. To explain the observed dynamic spectra of these chorus emissions, a possible generation mechanism is presented based on the recent nonlinear theory. On the basis of this theory, frequency sweep rate of chorus emission is computed and compared with that of our experimentally observed values, which shows, in general, good agreements.

The “trimpi event” consists of a sudden change in phase and amplitude of the VLF transmitter signal followed by a slow recovery to the initial levels as the ionization decays. We report here preliminary observations of phase and amplitude perturbations (trimpis) on VLF signals propagating in the Earth-ionosphere waveguide. Four VLF transmitter signals are monitored at our low latitude station Varanasi (L = 1.07) using SoftPAL Receiver. This paper deals with the current understanding of lightening discharge associated processes that leads to the changes in the characteristic of the waveguide and thus variation in the received amplitude and/or phase of the VLF transmission signals observed for the period August 25, 2009–December 25, 2009. Occurrence of Trimpi events at low L-values can be interpreted as precipitation of very high energetic electrons which are capable of producing perturbations during daylight, or as evidence of the ‘fast’ Trimpi mechanism which can dominate at low L-value. This wave induced particle precipitation is assumed to occur due to gyroresonance between whistler-mode waves and counter streaming electrons.

Large currents along the magnetic field transmit stresses between magnetosphere and ionosphere. If the electrons carrying such currents have high enough drift velocity, waves are generated. VLF waves play a major role in the Earth's magnetospheric dynamics. One of the advantages of ELF/VLF radio waves was their ability to propagate globally without excessive attenuation. Knowledge of wave characteristics and the current state of space weather can give one a good idea of what waves are likely to be occurring at any particular latitude and local time. In the present paper, I have emphasized the role of VLF waves as diagnostic tool for the remote sensing terrestrial magnetosphere. Many other aspects of these waves have been examined and results are associated with space weather related problems.

Various geomagnetic indices, viz. K_p, A_E, D_st, etc., and averaged solar wind IMF parameters govern the energy supply in certain regions of magnetosphere–ionosphere cavity. Recently Kozyreva et al. (2007) have developed a new ULF wave index to characterize the solar wind interaction and relativistic electron dynamics. Using data and model to derive the ULF wave index, I have shown that this index can be used as a tool in understanding the problems related to magnetospheric plasma dynamics. The variation of energy and momentum of relativistic electrons with ULF wave index causes the variability of IMF, geomagnetic field and solar wind plasma which are significant to study low frequency turbulence. The energy of relativistic electrons is not directly affected by the intensity of magnetic storm and has reciprocal relation with interval of these indices by the incident of elevated ULF pulsation in the magnetosphere. The possible applications of the ULF wave index in the space weather predictions are described in the present paper to develop a statistical approach to analyze various space weather related problems.

The paper represents first time observation of an unusual Multi-component natural event in the ELF range named “ELF whistlers” along with a precursor. Event is recorded during day time at a low latitude Indian station Jammu (geomag.lat. 19°26′N; L = 1.17). The detailed structure of this observed unusual event clearly show that such type of “multi-component ELF whistlers” with a low cutoff frequency, around 600Hz, well below the waveguide cutoff frequency along with a precursor emission has never been obtained at any of the mid and high latitudes during normal days where most VLF/ELF activity occurs. Generation and propagation mechanism of these events are discussed briefly. Plasma parameters are further derived from the dispersion analysis of whistlers.

Disturbances of the daily variations in the total electron content (TEC) of the ionosphere during and after geomagnetic storms, obtained from the observations of the GPS satellite signals, are considered. The specific features of these disturbances consist in the intensification of the variations with periods of 4–6 h, the amplitude of which increase at mid-latitudes, and in a weak dependence of local maximums on latitude and their considerable longitudinal variability. The possibility of explaining observed disturbances of the considered daily variations by the generation of standing planetary Poincare waves is discussed. The estimated periods of Poincare waves, latitudinal structure of these waves, and their ionospheric effects make it possible to qualitatively explain the observed specific variations in TEC.

The paper aims at understanding atmospheric system during earthquake preparatory processes through features of Total Electron Content (TEC) taken by GPS at Guwahati (26° 10′ N, 91° 45′ E), an anomaly crest station. It is noted that earthquakes occurring at equator (within same longitude zone of Guwahati) have little impact on pre-earthquake TEC variations over Guwahati, but the low-latitude ones show their imprints on TEC even when their epicenters lie at 500–600 km longitudinal separation. This paper discusses how electric field generated in the earthquake preparatory processes works in a complex manner at the equatorial anomaly zone in a way similar to electro jet current system, leading to changes in ionospheric parameters. Finally attempts are made to understand the situation prior to and during a seismic event by assigning parameters with special reference to IRI model, for obtaining the observed results.

The electrostatic charging of a satellite in space plasma is the result of charged particles impinging on or being ejected from the spacecraft. The resultant charge is a function of the properties of the spacecraft materials and the various sources of charged particles such as thermal electrons and ions, photoelectrons, secondary electrons, and energetic electrons of magnetospheric origin. On the other hand, theoretical and experimental studies on the dynamics of the interaction of moving bodies in plasma have demonstrated that the body also disturbs the plasma. In the present paper, we describe the two Langmuir probes designed and manufactured in Bulgaria, a part of the Plasma Wave Complex PWC (Obstanovka experiment) aboard the Russian segment of the International Space Station, whose goal is to monitor the surface charging and the noises and disturbances in the surrounding plasma induced by the station and by the experiments conducted on it.

A dispersion relation governing the magnetoacoustic surface waves on a tangential discontinuity (TD) is derived by taking into consideration the anisotropic proton pressure on either side of the TD. Theoretical understanding of the properties of various wave modes, as revealed by the dispersion relation, is then applied to specific examples of observed interplanetary TDs to arrive at simple estimates for the phase speed of important surface waves that may be suitable for future observations of such waves on arbitrary TDs.

We analyze the temporal image data of the on-disk coronal hole as observed on 31 March 2007 by the XRT/Hinode telescope and Al-poly filter. We choose this temporal image data of 30 s cadence from 11:34:48 UT to 14:19:35 UT to study intensity oscillations above the on-disk coronal hole, when the exposure time of each XRT image was 8.193 s. Using Fourier filtering method, we reconstruct X-ray light curve for the periods outside the cone-of-influence (COI) of its power spectrum. Using the standard wavelet software, we derive the power spectra of the reconstructed light curves which are generated by filtering the original X-ray time series at various Fourier scales outside the COI period. This procedure provides statistically significant and globally distributed multiple periodicities in the intensity and global wavelet power spectra. We observe the multiple periodicities above the on-disk coronal hole as 27.27 min, 13.64 min and 6.82 min. We interpret the first two observed periodicities as the most likely signature of magnetoacoustic wave harmonics that are probably excited in the lower solar atmosphere and leak through open field lines. The third periodicity is most probably generated due to recurrent magnetic reconnection at the boundaries of the on-disk coronal hole.

We study the solar source of intense geomagnetic storm of solar cycle 23 occurred on 20 November 2003. The flare/CME responsible for the geomagnetic storm originated from the super-active region NOAA 10501. We investigate the Hα observations of the flare event made with 15 cm Solar Tower Telescope at ARIES, Nainital, India. The propagation characteristics of the CME have been obtained from the three-dimensional images of the solar wind obtained from the interplanetary scintillation technique, supplemented with the other ground and space-based missions. It is inferred that two CMEs were originated from the same filament channel at the active region and their interaction along the Sun–Earth line has led to the severity of the storm. According to our investigation, the interplanetary medium consists of two merging magnetic clouds (MCs) that preserve their identity during their propagation. These magnetic clouds make the IMF southward for a long time, resulting the super-storm on the Earth.

We study the active region NOAA 10960, which produces two flare events (B5.0, M8.9) on 04 June 2007. We find the observational signature of right handed helical twists in the loop system associated with this active region. The first B5.0 flare starts with the activation of helical twist showing ∼3 turns. However, after ∼20 minutes another helical twist (with ∼2 turns) appears, which triggers M8.9 flare. Both helical structures were closely associated with a small positive polarity sunspot in the AR. We interpret these observations as evidence of kink instability, which triggers the recurrent solar flares.

Magnetic twist of the active region has been measured over a decade using photospheric vector field data, chromospheric H_α data, and coronal loop data. The twist and tilt of the active regions have been measured at the photospheric level with the vector magnetic field measurements. The active region NOAA 10930 is a highly twisted emerging region. The same active region produced several flares and has been extensively observed by Hinode. In this paper, we will show the evolution of twist and tilt in this active region leading up to the two X-class flares. We find that the twist initially increases with time for a few days with a simultaneous decrease in the tilt until before the X3.4 class flare on December 13, 2006. The total twist acquired by the active region is larger than one complete winding before the X3.4 class flare and it decreases in later part of observations. The injected helicity into the corona is negative and it is in excess of 10⁴³ Mx² before the flares.

Intense solar energetic particle (SEP) events data, associated with ground level enhancements (GLEs), occurred during 1989–2006 have been obtained from the spectrometers on board GOES spacecraft in the energy range 10–100MeV. The interplanetary effects of these events and their associated coronal mass ejections (CMEs) have been provided by the LASCO/SOHO coronagraph images in the field of view of 2–30R_⊙ and the interplanetary scintillation images from the Ooty Radio Telescope in the heliocentric distance range of ∼40–250R_⊙. The comparison between the radial evolution of the CME and its associated particle spectrum shows that the spectrum is soft at the onset of the particle event. A flat spectrum is observed at the peak of the particle event and the spectrum becomes steeper as the CME moves farther out into the inner heliosphere. However, the magnitude of change in spectral slopes differs from one CME to the other, suggesting the difference in energy available within the CME to drive the shock. The spectral index evolution as a function of initial speed of the CME at different parts of the particle profile has also been compared. The result shows that the reduction in particle flux with time is rather quick for the high-energy portion of the spectrum than that of the low-energy part, which makes the steepening of the energy spectrum with time/distance from the Sun. It indicates that the acceleration of particles by a CME-driven shock may be efficient at low energies (≤30MeV) and the efficiency of the shock decreases gradually towards the high-energy side of the spectrum.

Using data from observations made with XMM-Newton, we present an X-ray analysis of two flares observed in ξ Boo. The flare loop parameters are derived using various loop models including state-of-the-art hydrodynamic flare model. The loop lengths derived for the flaring loop structure are found to be less than the stellar radius. The exponential decay of the X-ray light curves, and time evolution of the plasma temperature and emission measure are similar to those observed in compact solar flares. The X-ray light curve of post flare phase is investigated with wavelet analysis. Wavelet analysis clearly show oscillations of the period of 1019 s. Using the observationally estimated loop length, density and magnetic field, the theoretically derived oscillation period for fast-kink mode approximately matches with the observationally estimated period. This is the first likely observational evidence of fundamental fast-kink mode of magnetoacoustic waves in the stellar loops during the post-flare phase of heightened emission.

Quasi-periodic oscillations in radio, X-rays, and optical emission from flaring stars are studied using the coronal seismology method. Two approaches are used: the first considers stellar coronal magnetic loops as resonators for MHD-oscillations, the second suggests that a coronal loop is an equivalent electric (RLC) circuit. It is shown that both approaches provide good diagnostic tools for parameters of flares on the Sun and late-type stars. A possible origin of high-frequency QPO from Soft Gamma Ray Repeaters is proposed.

The Sun, solar-like oscillators, δ-Scuti and roAp stars are few examples of the main-sequence pulsating stars for which the quantitative measurements of the internal structure parameters have been derived from their oscillations. For the asteroseismic study of δ-Sct and roAp stars, a project the “Nainital-Cape Survey” is being run at ARIES, Nainital since 1999. In this paper, we present the overview of stellar-seismology and briefly describe the results obtained under the survey project.